The invention concerns a digital indicating device, which consists of a liquid crystal that is placed between transparent plates. These plates are placed between two polarization filters and are arranged in a cell which is situated in a cabinet. The liquid crystal displays optical rotary power which can be controlled by an electric field. The portions of the plates of the cell corresponding to the character regions which are to be displayed have light-transmissive electrode pairs made from an electrically conducting material. Electrical fields perpendicular to the plates are established between selective electrode pairs responsive to an electronically controlled driving circuit.
In the known indicating devices of the above-mentioned type, the liquid crystal typically contains a nematic compound which causes the direction of polarization of the incident light to rotate 90.degree.. The polarization filters are crossed, so light from the surroundings which shines onto the device passes through the second polarization filter because of the activity of the liquid crystal; this light is reflected from a mirror placed behind this filter and returns through the cell to the eye of the observer. When an electric field is established between selected electrode pairs, the liquid crystal between these selected electrode pairs is deactivated and no longer rotates the light polarization. This causes no light to pass through the second polarization filter under the selected electrode pairs and as a result no light is reflected back to the observer. The character or numerical regions which correspond to these selected electrode pairs, therefore, appear as dark spots to the eye of the observer.
Indicating devices of the above-mentioned type, in principle, use ambient light from the surroundings e.g. daylight, so that a reading at night is not possible. Attempts have been made to illuminate the face of the cell with a radioactive light source and in so doing to simulate the surrounding light. The illumination intensities which are achieved in this way, are so weak that an easily readable display cannot be produced.
It would be plausible to set up a light source - in particular a radioactive one - in such a way, that the light from this source after passing through the initially-mentioned indicating device reaches the eye of the observer, which means operating the indicating device such that it transmits and does not reflect. However, in this case, considerable difficulties with respect to day and night display occur. If the displayed characters or figures appear as dark areas on a light background when exposed to the surroundings, this results in poor legibility at night.
The aim of the invention in question, is therefore, to create a digital indicating device of the foregoing type in which the physiologically favourable reading of the displayed characters is made possible, i.e., dark characters on a light background if the surroundings are light, and light characters on a dark background, if the surroundings are dark. The indicating device should be compact enough to install into a measuring or display instrument e.g., in electronic pocket calculators.